Solar cells have drawn public attention as an effective and environment-friendly energy source that can solve energy problems that have currently become more and more serious. At present, as a semiconductor material for use in photovoltaic elements for solar cells, inorganic substances such as monocrystalline silicon, polycrystalline silicon, amorphous silicon, and a compound semiconductor, have been used. However, since the solar cell to be manufactured by using inorganic semiconductors requires high costs compared to other power generation systems such as thermal power generation, it has not been widely used for general household purposes. The main reason for the high costs lies in that a process of forming a semiconductor thin-film requires high temperature and vacuum conditions. For this reason, organic solar cells have been investigated in which, as a semiconductor material expected to simplify the manufacturing process, an organic semiconductor and an organic dye such as a conjugated polymer and an organic crystal, are used. In such organic solar cells, the manufacturing process can be significantly simplified since the semiconductor material layer can be prepared by an application method.
However, the organic solar cells using the conjugated polymer or the like is lower in its photoelectric conversion efficiency than conventional solar cells using inorganic semiconductors and, therefore, those solar cells have not been put into practical use. To put the organic solar cell into practical use, it is essential to further improve photoelectric conversion efficiency.
As a method of improving the photoelectric conversion efficiency of the organic solar cell, a method in which when a photoelectric conversion layer composed of an electron donating organic semiconductor (e.g., a conjugated polymer) and an electron accepting organic semiconductor (e.g., a fullerene compound) is applied and formed, the photoelectric conversion layer is applied and formed by adding 1,8-diiodooctane to a solvent as an additive without using a single solvent, is often used (e.g., “Advanced Materials,” Vol. 22, p. E135-E138, 2010 and JP 5482973 B1).
When applying/forming a photoelectric conversion layer composed of the electron donating organic semiconductor and the electron accepting organic semiconductor, it is thought that the phase-separation structure is affected by affinity/repulsive properties among the electron donating organic semiconductor, the electron accepting organic semiconductor, a solvent and an additive, and is determined while these have complicated effects on one another. We believe that among the affinity, the affinity between the electron accepting organic semiconductor and the solvent, and the affinity between the electron accepting organic semiconductor and the additive are particularly important.
When the photoelectric conversion layer is applied/formed from an organic semiconductor solution, in a single solvent system, excessive aggregation of the electron accepting organic semiconductor or the like easily occurs in the process in which the organic semiconductor composition is dried/condensed after applying a solution because of insufficient affinity of the solvent for the electron accepting organic semiconductor. Therefore, a phase-separation size of the photoelectric conversion layer becomes excessively larger than 20 nm (as the phase-separation size, about 20 nm which is about two times as large as an exciton diffusion distance of an organic semiconductor (generally, about 10 nm) is thought to be suitable) and it has been difficult to adjust the photoelectric conversion layer to an appropriate size. Further, a solvent having high affinity for the electron accepting organic semiconductor, is excessively high in a boiling point and it has been difficult to apply the photoelectric conversion layer in an appropriate thickness in such a single solvent system.
A method of solving those problems includes a method of combining a solvent with an additive. As disclosed in the above-mentioned (“Advanced Materials,” vol. 22, p. E135-E138, 2010, it is reported that by adding a specific additive (1,8-diiodooctane) to a solution of forming a photoelectric conversion layer, a structure of phase-separation (phase-separation size, co-continuity, orientation) between the electron donating organic semiconductor and the electron accepting organic semiconductor varies to improve photoelectric conversion efficiency.
Moreover, when an additive can achieve the effect of improving performance even in small amounts, it is easy to obtain a cost advantage by combining the additive with a low-cost solvent. Further, since viscosity and drying property of a solution can be determined, it is easy to adjust characteristics of a composition according to an application method to be used. Further, even when a solvent is a high boiling point compound which is difficult to dry or such a compound that is solid at ordinary temperatures and pressures, a process advantage that drying becomes easy or it becomes possible to apply as a solution by adding a small amount of additive, is possible.
We confirmed that when the 1,8-diiodooctane is added as an additive in applying/forming a photoelectric conversion layer composed of the electron donating organic semiconductor and the electron accepting organic semiconductor, the photoelectric conversion efficiency is improved certainly. However, we believe that 1,8-diiodooctane has a high boiling point and low volatility resulting in slow drying, and thereby deterioration of processing properties and concerns about durability due to instability of a halogen compound become problems. Furthermore, we also believe that since 1,8-diiodooctane is a halogen compound, a non-halogen compound is preferred as an alternative to 1,8-diiodooctane also from the viewpoint of environmental load.
It could therefore be helpful to provide an organic semiconductor composition that can achieve high photoelectric conversion efficiency equal to the case of using 1,8-diiodooctane as an additive, and has good processing properties and low environmental load, and a photovoltaic element using the organic semiconductor composition.